Jonathan Swift, John Hawkesworth, Deane Swift (1768). “Letters Written By The Late Jonathan Swift, D. D. Dean of St. Patrick's, Dublin; And Several Of His Friends: From The Year 1703 To 1740”, p.174
Letter to Duc de Richelieu, 18 June 1744. Although this saying is now associated with Voltaire, he is obviously quoting an Italian proverb here. The French form, which he used later, is Le mieux est l'ennemi du bien.